T-Scores

• T-scores are ‘Transformed Scores’ used to standardise raw scores across the level or cohort. • T-scores give the relative rank or position of a pupil’s performance compared to all the other pupils in that subject in the same exam. • T-scores can be computed for individual subjects and then summed up as a single aggregate T-score, say in the case of PSLE. • T-scores allow pupils to be ranked fairly.

How are T-Scores calculated in PSLE?

• As mentioned, T-score is the transformed or adjusted score a student will get for a subject, after factoring in the level or cohort’s mean and SD for that subject.

• Formula for T-ScoreX = Raw score of student in the subjectY = Average or Mean Score of the level or cohortZ = Standard Deviation (SD) of the level or cohort Standard Deviation (SD) is the spread of the marks around the average.

Why Standard Deviation (SD) matters?

• Here is an example to illustrate that with the same mean, SD or the spread of marks can vary.• Example 1 – Allan, Bernard and Charles have $45, $50 and $55 respectively. They have an average of $50 each.

Example 2 – Dan, Edward and Frank have $10, $50 and $90 respectively. They also have an average of $50 each.

• In Example 1, the spread (from $45 and $55 to average of $50) is smaller than the spread• in Example 2 (from $10 and $90 to average of $50)• Conclusion: Despite having the same mean, Example 1’s data have a smaller SD or “spread from the mean” as compared to that of Example 2